Various integrated process control systems have been designed to automate factory machinery, processes in municipal water or sewage treatment plants, commercial buildings, etc. These systems are designed to transmit a significant number of system status monitoring and process control functions for display on computer monitors, database storage and process performance reports. Continuous monitoring of various equipment and process streams is required to establish optimum operating parameters, such as determining when and how an alarm should alert operators or plant managers to changes in conditions, controlling of various equipment and process systems and the like. The objective of a process control system is to collect information on all the appropriate parameters, e.g., system or line pressures, flow, temperature and control the appropriate equipment, e.g., motors, fans, pumps and valves, to maintain the proper operating conditions or transmit an alarm signal if necessary.
One type of control system currently in use was designed to automate factory machinery. An individual stand-alone integrated Programmable Logic Controller (PLC) uses a microprocessor, sensor inputs and control outputs. The PLC processes the incoming sensor signals and transmits programmed electronic signals to the process equipment being controlled. Such PLCs can be integrated into a network linking a group of remote PLCS together to coordinate the operation of a factory production line.
Another type of control system is designed to typically monitor an entire water or sewage treatment plant or a community-wide network of mechanical or electronic devices. A common approach to plant process control uses a central master computer with specific software to monitor and control an array of module drivers inside a Remote Terminal Unit (RTU). The modules collect and transmit sensor data in the form of analog signals and in turn actuate switches, solenoids and the like to control process equipment. These modules perform simple functions in either acquiring or processing sensor data or generating the appropriate analog signal outputs using software driver codes written specifically for each individual module driver address. A supervisory control and data acquisition module array inside the RTU enclosure is connected by a serial link with a central computer.
Typically, the sensors used in these systems produce a voltage, current or change in resistance or capacitance depending on the quantity sensed, such as optical, pressure or magnetic phenomena. Where the sensors produce a voltage output, the voltage produced at different parameter levels and the sensor sensitivity in volts per unit of measured quantity may vary from sensor to sensor, particularly where the sensors are of different types or made by different manufacturers. For example, in a pressure sensing system, it may be desirable to have the sensor read 0 volts when no pressure is applied and 5 volts when 100 psi is sensed, and the sensor voltage level output should track changes in pressure. Since it is important to have all sensors in a system to "look" the same to a central control unit, conventionally the sensor circuitry is "trimmed" with potentiometers or laser trimming of resistors to correct for offsets from the desired voltage and sensitivity changes which often differ from sensor to sensor. However, such trimming may not provide precise matching, offsets may change with time, temperature etc. and with different types of sensors trimming is often not effective.
Therefore, there is a continuing need for improved process control systems for complex processes and provision for more convenient determination and application of any needed sensitivity or signal output offsets to provide improved uniformity between sensors in the system and to permit different types of sensors to be used in a sensor array.